Don't do it.
It's mostly seasonal and you don't make enough in a season to get by the rest of the year, even if you have a second job.
Also be prepared to scrub toilets and deal with stupid people if you are a ranger. You will rarely get into the true back country.
Also, assuming you are a white male, your chances are low at getting hired by any federal agency. They give high priority to women and minorities.
Find a job doing something else where you make a lot of money and get time off to go see cool places on your own time.
but if you're still adamant about doing so, look for jobs here:
usajobs.com 
